Cement composition and methods for producing same

ABSTRACT

A cement composition comprising 1) a cement containing water-soluble chromate and 2) at least one manganese(II) compound in an amount sufficient to reduce the amount of water-soluble chromate to at the most 2 mg of Cr(VI) per kg of the cement, the determination of the chromate content being carried out by elution of the composition in water for 15 minutes at a water/cement ratio 1, filtration and determination of the chromate content in the filtrate.

FIELD OF THE INVENTION

The present invention relates to a cement composition with reducedcontent of water-soluble chromate, a method for reducing the amount ofwater-soluble chromate in a cement composition, and to methods forproducing a cement composition with reduced content of water-solublechromate.

BACKGROUND OF THE INVENTION

It is well known that water-soluble chromate in cement gives rise toallergic reactions in workers subjected to contact withcement-containing products. In particular, construction workers who aresubjected to regular skin contact with wet cement such as wet cementpaste, wet mortar, and concrete mixes, have a risk of contractingchromate-related eczema.

It is known to reduce the content of water-soluble chromate in a cementcomposition by addition of ferrous sulphate. The ferrous sulphate may beadded, e.g., during the preparation of cement-containing mixes or duringmanufacture of the cement, vide e.g. WO 82/02040. The iron(II) sulfatereduces Cr⁺⁶ to Cr⁺³, the latter having low solubility in cement-watermixes. The reaction between Fe⁺² and Cr⁺⁶ takes place in aqueoussolution, e.g. when water is added to a cement containing iron(II)sulfate.

SUMMARY OF THE INVENTION

It has now been found that manganese(II) compounds are effective forreducing the content of water-soluble chromate in cement compositions.Manganese(II) compounds, even in technical grade, have the advantagethat they can be easily milled or ground to a small particle size wellsuited for addition to cement or cement compositions to obtain asatisfactorily homogeneous distribution. Some manganese(II) compoundssuch as manganese(II) sulfate have the advantage that they are oxidationstable in dry cement compositions even at high temperature, areavailable as dry free-flowing powders in technical grades and have highchromate-reduction efficiency when they are interground with cement.

DETAILED DISCLOSURE OF THE INVENTION

Thus, in one aspect, the invention relates to a cement compositioncomprising 1) a cement containing water-soluble chromate and 2) at leastone manganese(II) compound in an amount sufficient to reduce the amountof water-soluble chromate to at the most 2 mg of Cr(VI) per kg of thecement, the determination of the chromate content being carried out byelution of the composition in water for 15 minutes at a water/cementratio of 1, filtration, and determination of the chromate content in thefiltrate.

The cement is one which, if no measures are taken, containswater-soluble chromate in amounts which are objectionable from a healthhazard point of view, typically amounts of above 2 mg of Cr(VI) perkilogram of the cement, determined as described above, thisdetermination preferably being performed in accordance with DanishStandard DS 1020. In this standard method, an agent which forms acoloured complex with chromate, i.e. s-diphenyl carbazide, is added tothe filtrate obtained following elution of the cement, whereupon theintensity of the coloured complex is measured spectrophotometrically,and converted into content of water-soluble chromate by means of acalibration curve. As will be understood, the content of water-solublechromate in the filtrate may also be measured in other suitable manner.

It is normally preferred that the manganese(II) compound is present inthe composition in an amount sufficient to reduce the amount ofwater-soluble chromate to at the most 1 mg, preferably at the most 0.5mg, in particular at the most 0.1 mg, especially at the most 0.01 mg ofCr(VI) per kg of the cement component.

The amount of manganese(II) compound sufficient to obtain the abovereduction in the content of water-soluble chromate will typically be anamount representing a molar excess, calculated on the chromate, andthus, the minimum amount of manganese(II) compound is dependent on thenatural content of water-soluble chromate in the cement in question.However, as a guideline, the manganese(II) compound may be present in anamount corresponding to from 1 to 1000 mmol manganese(II) per kg of thecement component, in particular an amount corresponding to 5-100 mmolmanganese(II) per kg of the cement, preferably 10-60 mmol per kg of thecement.

The manganese(II) compound may be a manganese(II) salt; a saltcontaining manganese(II), a partly oxidized manganese(II) salt; aproduct containing manganese(II) such as particles containingmanganese(II) or a manganese(II)-containing amorphous phase, e.g. aglass phase.

As suitable examples of the manganese(II) compound may be mentioned asalt with a mineral acid or with an organic acid. The mineral acid saltmay be the sulfate, hydrogen sulfate, chloride, bromide, carbonate,nitrate, nitrite, sulfite, sulfide, disulfide, dithionate, thiosulfate,thiocyanate, hydroxide, oxide, monohydrogen phosphate, dihydrogenphosphate, silicate or hydrates thereof, preferably the sulfate. It mayalso be a double salt such as ammonium manganese(II) sulfate orpotassium manganese(II) sulfate. The organic acid salt may, e.g., be acarboxylic acid salt such as a salt with an alkanoic acid such as formicacid or acetic acid, or a salt with a hydrocarboxylic acid such aslactic acid, tartaric acid, citric acid, gluconic acid or malic acid ora salt with a sulfonic acid such as methylsulfonic acid, a naphtalenesulfonic acid or a melamine sulfonic acid. Another example of amanganese(II)-containing compound is manganese(II) oxide sulfate.

The manganese(II) compound may typically be manganese(II) sulfate suchas a technical grade manganese(II) sulfate, preferably containing morethan 20%, in particular more than 25%, especially more than 31% byweight of manganese, and it may be hydrated, such as monohydrated orpartially monohydrated. Manganese(II) sulfate has several advantages,i.a. that it is available as a dry free-flowing powder even in technicalgrades, that technical grades are available as a fine powder, that it isoxidation stable even at high temperature, that it is cheap since it isavailable as a byproduct from the preparation of hydroquinone, and thatit has high efficiency as a chromate-reducing agent for cementparticularly when it is interground with cement. Furthermore,manganese(II) sulfate is not very hygroscopic and is available asMnSO₄.H₂ O in technical grades.

The manganese(II) compound may also be a manganese(II)-containingmineral, e.g. szmikite, rhodochrosite, manganosite, alabandite, punite,manganosiderite, oligonite, or manganolangbeinite.

A suitable manganese(II) compound may be a soluble manganese(II) salt,e.g. manganese(II) sulfate or manganese(II) acetate but themanganese(II) compound may also be of low solubility, e.g. manganese(II)carbonate. It is preferable that a manganese(II) compound with lowsolubility has a high specific surface area. This may for example beobtained by intergrinding the manganese(II) compound with the cement.

Manganese(II) sulfate is a soluble salt with a solubility of 985 gMnSO₄.H₂ O per liter of solution at 48° C. Even though manganese(II)sulfate is a soluble salt it has surprisingly been found that theefficiency can be increased by fine-grinding. Thus, in the cementcomposition of the invention, a reduced dosage of manganese(II) sulfateis required to reduce water soluble chromate when the manganese(II)sulfate is fine-ground.

Fine-grinding of the manganese(II) sulfate as well as othermanganese(II) compounds may be done by intergrinding the manganese(II)compound with cement to a specific surface of the cement compositionaccording to Blaine of more than 250 m² /kg preferably more than 300 m²/kg and most preferably more than 400 m² /kg.

In a preferred embodiment, the manganese(II) compound is manganese(II)sulfate which is present in the cement composition in an amount of0.01-10% by weight, in particular 0.1-1% by weight of cement.

It is especially interesting to note that suitable manganese(II)compounds may be ground or milled to a small particle size which mayeasily be distributed evenly in a dry composition, e.g. a mean particlesize below 0.1 mm, preferably below 0.05 mm, in particular below 0.02mm.

The cement may be any cement which contains a water-soluble chromate inan unacceptable amount. Common examples of such cements are Portlandcements such as ordinary Portland cement, rapid hardening cement, andsuper rapid hardening cement; belite cement; low heat cement; blendedcements; slag cements; Pozzolan cements; and the like, in particularPortland cements and blended cements.

In addition to the cement and the manganese(II) compound, thecomposition of the invention may further contain at least one materialselected from aggregate material, reinforcing material, water, slakedlime, and cement and concrete additives.

The aggregate material may, e.g., be selected from fine aggregate suchas sand, and coarse aggregate such as stone.

The cement or concrete additive may, e.g., be a retarding agent, aconcrete plasticiser or superplasticiser, or an air entraining agent.

The composition of the invention may be in dry or wet form. A preferredcomposition is a composition in dry form wherein the manganese(II)compound is in the form of particles having a particle size as mentionedabove.

The invention also relates to a method for reducing the amount ofwater-soluble chromate in a cement composition, said method comprisingadding to the composition during the manufacture thereof a manganese(II)compound in an amount sufficient to effect a reduction in the content ofwater-soluble chromate to at the most 2 mg of Cr(VI) per kg of thecement, in particular at the most i mg, preferably at the most 0.5 mg,Cr(VI) per kg of the cement, the determination of the chromate contentbeing carried out as mentioned above.

The addition of the manganese(II) compound may be carried out in anumber of ways and at any of a number of stages of the preparation ofthe cement or the cement-containing composition.

Thus, the addition of the manganese(II) compound may, e.g., be carriedout

1) by addition to cement clinker prior to or during grinding thereof;

2) by addition to semi-ground cement clinker;

3) by addition to a component added during the production of cement;

4) by addition of the compound as a concentrated aqueous solution toclinker in such a manner that the solution is evaporated;

5) by addition to cement powder prior to or during transfer thereof to astorage container or to a transport vehicle or prior to or duringpackaging thereof;

6) by addition to cement powder during transfer thereof from a transportvehicle to a storage container;

7) by addition to cement powder or a composition containing cementpowder prior to mixing thereof with water;

8) by addition of the compound as a solution in cement mixing water; or

9) by addition of the compound in dry form or as an aqueous suspensionor solution prior to, during or after the mixing of cement powder or acomposition containing cement powder with water.

When the manganese(II) compound is added to cement clinker prior to orduring grinding of the clinker or to semi-ground clinker, the additionmay, e.g., be performed by means of a pneumatic transport system wherethe dosage of manganese(II) compound is controlled by means of a wormtransporting device.

Addition of the manganese(II) compound to a component added during theproduction of cement may be by addition in admixture with e.g. gypsum,fly ash, slag, limestone, or similar components.

The addition of the manganese(II) compound as a concentrated solution toclinker in such a manner that the solution is evaporated may beperformed by adding the solution to the clinker shortly after the rotarykiln while the clinker are still warm, or during the grinding of theclinker where the clinker are heated mechanically.

Addition of the manganese(II) compound to a cement or a cementcomposition during transfer thereof to a storage container or atransport vehicle, prior to packaging thereof, or during transferthereof from a transport vehicle to a storage container may be performedby means of well-known techniques. Thus, the cement or cementcomposition may typically be transferred by means of a pneumatictransport system which incorporates a system for pneumatic addition ofthe manganese(II) compound in a well-known manner to the stream ofcement or cement composition during the transfer thereof.

When adding the manganese(II) compound to cement or cement compositionprior to the mixing thereof with water, the manganese(II) compound maybe mixed in dry form with the cement or cement composition to give ahomogeneous mixture. A particularly interesting example of this aspectis the addition of a manganese(II) compound during the manufacture of adry mortar mix consisting of cement powder, sand, slaked lime andconcrete additives.

When adding the manganese(II) compound as a solution in cement mixingwater, the concentration of manganese(II) compound is adjusted accordingto the amount of manganese(II) compound and the amount of waterrequired. Alternatively, the solution of manganese(II) compound may bemore concentrated and constitute only part of the total amount of waterrequired.

It may be summarised that when conducting the addition of amanganese(II) compound to a cement or cement composition at e.g. one ofthe above outlined steps, the manganese(II) compound may be supplied tothe cement or cement composition as a wet powder; a solution; aconcentrated solution; a dry, free-flowing powder; or mixed orinterground with an inert powder to facilitate handling.

The invention is further illustrated by the following non-limitingexamples.

EXAMPLE 1

Methods

Determinations of water-soluble chromate were carried out according toDanish Standard DS 1020 as described in detail below.

Measurements of the total reduction capacity of the cement mixes werealso carried out. The reduction capacity is a measure of the excessreducing capacity present in the mixes, i.e. the ability to reduce addedchromate. The reduction capacity was determined by extracting the cementmixes with solutions of potassium dichromate containing 50 and 100 mg ofCr⁺⁶ pr. kg solution. The cement was extracted for 15 minutes at awater/cement ratio of 1. The reduction capacity was calculated by thefollowing formula: ##EQU1##

M50=chromate content in filtrate after elution with a solutioncontaining 50 mg of Cr⁺⁶ /kg.

M100=Chromate content in filtrate after elution with a solutioncontaining 100 mg of Cr⁺⁶ /kg.

Test method for water soluble chromate in cement according to DanishStandard DS 1020 (First Edition, July, 1984)

Method principle

The cement is eluted with water and filtered. To part of the filtrate,s-diphenyl carbazide is added which together with chromate in an acidmedium forms a reddish-violet complex with an absorption maximum at 540nm. The colour of the complex is measured photometrically and convertedinto content of water-soluble chromate by means of a calibration curve.

Reagents

All chemicals must be analytical grade. For the preparation of reagentsand dilutions, chromate-free distilled or demineralized water is used.

Potassium permanganate, 0.02 M: 0.3 g of potassium permanganate (KMnO₄)is dissolved in 100 ml of water.

Sulphuric acid, 1.8 M: 96 ml of concentrated sulphuric acid (H₂ SO₄,d=1.84 g/ml) are added to approx. 900 ml of water and diluted with waterup to 1000 ml. Potassium permanganate, 0.02 M, is added until the colouris slightly pink.

Ethanol (C₂ H₅ OH, d=0.79 g/ml).

Indicator solution: 0.125 g of s-diphenyl carbazide [(C₆ H₅ NHNH)₂ CO,1,5-diphenyl carbohydrazide] is dissolved 25 ml of ethanol in a 50 mlvolumetric flask. Water is added up to 50 ml. The indicator solution canbe considered stable for up to 3 hours.

Chromate stock solution, 50 mg of Cr⁺⁶ per litre: 0.1414 g of driedpotassium dichromate (K₂ Cr₂ O₇) is dissolved in water in a 1000 mlvolumetric flask and diluted with water up to 1000 ml.

Chromate standard solution, 5 mg of Cr⁺⁶ per litre: 50.0 ml of chromatestock solution are measured into a 500 ml volumetric flask and dilutedwith water up to 500 ml. The standard solution must not be stored.

Apparatus

Magnetic stirrer with plastic-coated magnets or shaking apparatus.

Filter crucible with a capacity of about 60 ml and a porosity of 4.

Vacuum pump and suction flask for filter crucible or other equipment.

Spectrophotometer for measuring at a wavelength of 540 nm or a filterphotometer with a filter providing maximum transmission around 540 nm.

Cuvette with a light path of 10 mm.

Measuring procedure

Calibration

Calibration solutions: 1.0, 2.0, 5.0, 10.0 and 15.0 ml of chromatestandard solutions are transferred to 50 ml volumetric flasks. Thecalibration solutions contain 5.10, 25, 50 and 75 μg of Cr⁺⁶,respectively. To each flask, 5 ml of sulphuric acid, 1.8 M, are addedfollowed by dilution with water to a volume of about 40 ml. The solutionis cooled to room temperature, 5 ml of indicator solution are added, andthe flask is filled up to volume with water and shaken. The absorbanceof the solution is measured with the reagent blind sample describedbelow as the reference at a wavelength of 540 nm 15-30 minutes after theaddition of the indicator solution.

Calibration curve: The calibration curve is drawn by plotting themeasured absorbance values versus the content of Cr⁺⁶. The calibrationcurve must be linear and should be checked regularly and alwaysfollowing exchange of reagents.

Analysis

The reagent blind sample is prepared as described above but using waterinstead of standard solution.

Elution: 25.0 g of cement are transferred to a 100 ml beaker and 25.0 mlof water added. The slurry is agitated vigorously with a magneticstirrer or a shaker for 15±1 minutes. The slurry is filtered through adry filter crucible into a dry suction flask.

Measurement: An analysis sample is made in the same manner as thecalibration solutions but using 5.0 ml of filtrate instead of standardsolution, and the measurement is carried out as described above.

Results

Calculation: The content of Cr⁺⁶ in the solution is read off from thecalibration curve. The content of water-soluble chromate in the cementis calculated from the formula

x=y/5

in which

x=the content of water-soluble chromate in the cement expressed as mg ofCr⁺⁶ per kg of cement.

y=the content of Cr⁺⁶ (μg) in the solution, read off from thecalibration curve.

For the final result, the mean of two determinations not varying morethan 0.4 mg/kg from one another is used.

In this example, manganese(II) sulfate was interground withchromate-containing cement clinker and gypsum to investigate the abilityof manganese(II) sulfate to reduce water soluble chromate in cement.

The following materials were used:

Manganese(II) sulfate of technical grade of Chinese origin. It containedmore than 31% manganese and it was obtained from Superfos KemikalierA/S, Denmark. MnSO₄.H₂ O and MnSO₄ was detected by examination withX-ray diffraction.

Gypsum of technical grade from Boliden, Sweden.

Cement clinker manufactured by Norcem A.S., Norway. The chemicalcomposition of the clinker was:

    ______________________________________                                                SiO.sub.2                                                                           20.61%                                                                  Al.sub.2 O.sub.3                                                                    4.99%                                                                   Fe.sub.2 O.sub.3                                                                    3.55%                                                                   CaO   64.81%                                                                  MgO   2.46%                                                                   SO.sub.3                                                                            1.68%                                                                   K.sub.2 O                                                                           1.30%                                                                   Na.sub.2 O                                                                          0.38%                                                           ______________________________________                                    

Water soluble chromate: 28.3 mg of Cr⁺⁶ /kg (Determined by elution with20% sodium sulfate solution for 15 minutes at a water to clinker ratioof 1)

Cement clinker, 5% gypsum and varying amounts of manganese(II) sulfatewas interground in a laboratory mill to a specific surface according toBlaine of 410-510 m² /kg. Propylene glycol was added as a grinding agentin a dosage of 0.05%.

The content of water-soluble chromate was determined according to DanskStandard DS 1020 (First Edition, July, 1984).

Measurements of the reduction capacity of the cement mixes were alsocarried out.

These measuring methods were also used in examples 2-5.

The results of the measurements are given in Table 1 below.

                                      TABLE 1                                     __________________________________________________________________________                 Water              Blaine                                                     soluble                                                                             Chromate     surface of                                             Dosage                                                                            chromate                                                                            in filtrate                                                                         Reduction                                                                            the cement                                    Reducing agent                                                                         (%) (mg Cr.sup.+6 /kg cement)                                                                 capacity                                                                             (m.sup.2 /kg)                                 __________________________________________________________________________    Chromate in  0.0   50 100                                                     added water                                                                   (Plain cement)                                                                         0.00                                                                              19.7* 67 112                                                                              -23    506                                           Manganese (II)                                                                         0.25                                                                              2.9   32 73 11     500                                           sulfate, 0.30                                                                              0.2   25 62 16     454                                           technical grade                                                                        0.40                                                                              0.1   18 50 21     437                                                    0.50                                                                              0.0    1 17 48     492                                                    0.75                                                                              0.0    0  0 >50    500                                           MnSO.sub.4.H.sub.2 O,                                                                  0.25                                                                              7.4   40 80  1     417                                           analytical grade                                                                       0.50                                                                              0.0    0  1 >50    498                                           __________________________________________________________________________     *The content of water soluble chromate in the plain cement was 25.0 mg        Cr.sup.+6 /kg when the determination was carried out by elution with 20%      sodium sulfate solution.                                                 

The results show that manganese(II) sulfate when interground with cementis an efficient chromate reducing agent and that a low dosage ofmanganese(II) sulfate is sufficient to reduce all water-solublechromate.

EXAMPLE 2

Manganese(II) sulfate was mixed with chromate containing cement toinvestigate the ability of unground manganese(II) sulfate to reducesoluble chromate in cement.

The manganese(II) sulfate of technical grade and the plain cement fromexample 1 were used. The manganese(II) sulfate was a free flowing whitepowder with a 0.045 mm sieve residue of 48%.

The cement mixes were prepared by mixing plain cement and manganese(II)sulfate in a plastic bucket with a paint mixer for 3 minutes.

The content of water-soluble chromate and the reduction capacity wasmeasured.

The results of the measurements are given in Table 2 below.

                  TABLE 2                                                         ______________________________________                                                      Water                                                                         soluble   Chromate                                                      Dosage                                                                              chromate  in filtrate                                                                              Reduction                                  Reducing agent                                                                          (%)     (mg Cr.sup.+6 /kg cement)                                                                      capacity                                   ______________________________________                                        Chromate in       0.0       50   100                                          added water                                                                   (Plain cement)                                                                          0.00    19.7      67   112   -23                                    Manganese (II)                                                                          0.25    13.4      53   96    -13                                    sulfate,  0.50    7.0       39   76     -3                                    technical grade                                                                         1.00    0.7       19   45     16                                              1.50    0.0        3   20     43                                              2.00    0.0        0    5    >50                                              2.50    0.0        0    2    >50                                    ______________________________________                                    

The results show that unground manganese(II) sulfate is able to reducewater soluble chromate in cement. When the results of examples 1 and 2are compared it can be seen that a lower dosage of manganese(II) sulfateis needed to reduce the water soluble chromate to less than 2 mg of Cr⁺⁶/kg when the manganese(II) sulfate is interground with cement than whenit is only mixed with the cement.

EXAMPLE 3

Manganese(II) sulfate, manganese(II) carbonate and iron(II) sulfate wereinterground with chromate-containing cement clinker and gypsum toinvestigate the ability of these compounds to reduce water solublechromate in cement.

The following materials were used:

Manganese(II) sulfate of technical grade from example 1.

Manganese(II) carbonate hydrate, MnCO₃.xH₂ O, of analytical grade.

Iron(II) sulfate monohydrate, FeSO₄.H₂ O prepared by drying ofanalytical grade FeSO₄.7H₂ O in a nitrogen atmosphere at 120° C.

Iron(II)sulfate heptahydrate, FeSO₄.7H₂ O, of analytical grade.

Cement clinker from Aalborg Portland produced in kiln 87 on Nov. 21,1990. The content of water soluble chromate was 6,2 mg of Cr⁺⁶ /kg.(Determined by elution with 20% sodium sulfate solution for 15 minutesat a water to clinker ratio of 1).

Gypsum of technical grade from Boliden, Sweden.

Cement clinker, 5% gypsum and varying amounts of chromate reducing agentwere interground in a laboratory mill to a specific surface of 390-530m^(2/) kg. Propylene glycol was added as a grinding agent in a dosage of0.05%.

The content of water soluble chromate and the reduction capacity wasmeasured.

The results of the measurements are given in Table 3 below.

                                      TABLE 3                                     __________________________________________________________________________                 Water              Blaine                                                     soluble                                                                             Chromate     surface of                                             Dosage                                                                            chromate                                                                            in filtrate                                                                         Reduction                                                                            the cement                                    Reducing agent                                                                         (%) (mg Cr.sup.+6 /kg cement)                                                                 capacity                                                                             (m.sup.2 /kg)                                 __________________________________________________________________________    Chromate in  0.0   50 100                                                     added water                                                                   (Plain cement)                                                                         0.00                                                                               3.2* 44 85 -5     486                                           Manganese (II)                                                                         0.25                                                                              0.0   16 48 25     460                                           sulfate, 0.50                                                                              0.0    1 15 48     403                                           technical grade                                                               MnCO.sub.3.H.sub.2 O,                                                                  0.25                                                                              1.7   42 82 -3     498                                           analytical grade                                                                       0.50                                                                              0.0   32 71  9     496                                                    1.00                                                                              0.0   33 73  9     430                                                    2.00                                                                              0.0    0  0 50     530                                           FeSO.sub.4.H.sub.2 O                                                                   0.25                                                                              0.0   20 61 25     395                                                    0.50                                                                              0.0    0 34 50     406                                           FeSO.sub.4.7H.sub.2 O,                                                                 0.25                                                                              0.0    0 25 >50    429                                           analytical grade                                                                       0.50                                                                              0.0    0  0 >50    493                                           __________________________________________________________________________     *The content of water soluble chromate in the plain cement was 5.1 mg of      Cr.sup.+6 /kg when the determination was carried out by elution with 20%      sodium sulfate solution.                                                 

The results show that manganese(II) sulfate is an efficient chromatereducing agent when it is interground with cement. Manganese(II)carbonate was also able to reduce chromate when interground with cement.

EXAMPLE 4

The cements from example 3 containing manganese(II) sulfate and iron(II)sulfate were heated to temperatures of 100°, 150°, 200°, 250° and 400°C. to determine the ability of the cements to withstand elevatedtemperatures.

Heat treatment of the cements were carried out by placing 100 g of thecement in aluminium trays without lid and placing the aluminium traysfor 3 hours at the desired temperatures.

Cement which was heat-treated at 400° C. was mixed with 4% calciumsulfate hemihydrate before elution for determination of water solublechromate and reduction capacity.

The content of water soluble chromate in the cements and the reductioncapacity was measured.

The results of the measurements are given in Table 4 below.

                                      TABLE 4                                     __________________________________________________________________________    Cements heated to 100, 150, 200, 250 and 400° C. for 3 hours                        Temperature                                                                          Water                                                                  of heat                                                                              soluble                                                                            Chromate                                                     Dosage                                                                             treatment                                                                            chromate                                                                           in filtrate                                                                          Reduction                                     Reducing agent                                                                        (%)  (°C.)                                                                         (mg Cr.sup.+6 /kg cement)                                                                 capacity                                      __________________________________________________________________________    Chromate in         0.0  50  100                                              added water                                                                   (Plain cement)                                                                        0.00 100    3.4  48  88 -4                                                         150    3.6                                                                    200    3.8                                                                    250    3.9                                                                    400    4.5                                                       Manganese (II)                                                                        0.25 100    0.0  13  43 29                                            sulfate,                                                                              0.25 150    0.0   6  40 40                                            technical grade                                                                       0.25 200    0.0  13  44 29                                                    0.25 250    0.0  18  48 18                                                    0.25 400    0.0  22  71 27                                                    0.50 100    0.0   0   7 >50                                                   0.50 150    0.0   0   6 >50                                                   0.50 200    0.0   0   8 >50                                                   0.50 250    0.0   0   0 >50                                                   0.50 400    0.0  15  46 25                                            FeSO.sub.4.H.sub.2 O                                                                  0.25 100    0.0  19  59 27                                                    0.25 150    0.0  18  60 28                                                    0.25 200    0.0  29  71 15                                                    0.25 250    0.0  40  80  1                                                    0.25 400    0.2  40  83  2                                                    0.50 100    0.0   0  25 >50                                                   0.50 150    0.0   0  26 >50                                                   0.50 200    0.0  13  54 34                                                    0.50 250    0.0  24  66 22                                                    0.50 400    0.0  32  73 22                                            FeSO.sub.4.H.sub.2 O,                                                                 0.25 100    0.0   7  47 41                                            analytical grade                                                                      0.25 150    0.0  23  62 22                                                    0.25 200    0.0  30  70 13                                                    0.25 250    0.0  41  84 -2                                                    0.25 400    4.5                                                               0.50 100    0.0   0   7 >50                                                   0.50 150    0.0  15  52 30                                                    0.50 200    0.0  19  60 27                                                    0.50 250    0.0  28  69 15                                                    0.50 400    3.2                                                       __________________________________________________________________________

In industrial cement milling systems, the temperature is normallyelevated due to release of heat from the grinding process and due toresidual heat in the clinker.

The results show that manganese(II) sulfate has a better ability towithstand elevated temperatures than iron(II) sulfate. When the chromatereducing agent is added during cement grinding the dosage which isnecessary to reduce water soluble chromate may be lower formanganese(II) sulfate than for iron(II) sulfate.

EXAMPLE 5

Manganese(II) chloride, manganese(II) acetate, manganese(II) nitrate andmanganese(II) sulfate were mixed with chromate-containing cement todetermine the ability of these manganese(II) salts to reduce watersoluble chromate in cement. Furthermore, manganese(II) sulfate was addedin dissolved form to the cement.

The following materials were used:

Manganese(II) chloride, MnCl₂.4H₂ O, of analytical grade

Manganese(II) acetate, Mn(C₂ H₅ O₂)₂.4H₂ O, of analytical grade

Manganese(II) nitrate, Mn(NO₃)₂.4H₂ O, of analytical grade

Manganese(II) sulfate, MnSO₄._(H2) O, of analytical grade

Manganese(II) sulfate of technical grade from example 1.

Ordinary Portland cement of Polish origin with a water soluble chromatecontent of 9.4 mg of Cr⁺⁶ /kg.

Mixes of the cement and the manganese(II) salts were prepared by mixingcement and unground manganese(II) salts in a paint mixer for 1 minute.

The content of water soluble chromate was determined according to DanskStandard DS 1020 the same day as the mixes were prepared and again afterstanding for 14 days.

The following procedure was used when manganese(II) sulfate was added tothe cement in dissolved form. Solutions containing 0.05%, 0.10% and0.25% MnSO₄. H₂ O of analytical grade were made and immediately used forelution of the cement. Determination of chromate were carried outaccording to Dansk Standard DS 1020 except that the manganese(II)sulfate solutions were used for elution instead of pure water.

The results of the measurements are given in Table 5 and 6 below.

                  TABLE 5                                                         ______________________________________                                        Water soluble chromate (mg Cr.sup.+6 /kg) in cement mixes                     ______________________________________                                        Dosage           Mn(C.sub.2 H.sub.5 O.sub.2).sub.2.4H.sub.2 O                 (%)   MnCl.sub.2.4H.sub.2 O                                                                    (mg Cr.sup.+6 /kg)                                                                           Mn(NO.sub.3).sub.2.4H.sub.2 O                 ______________________________________                                        0.10  1.3        3.7            9.1                                           0.25  0.0        0.0            2.7                                           0.50  0.0        0.0            0.9                                           0.75                            0.0                                           ______________________________________                                                         Manganese (II)                                                                sulfate,                                                     Dosage           technical grade                                                                            MnSO.sub.4.H.sub.2 O dissolved                  (%)   MnSO.sub.4.H.sub.2 O                                                                     (mg Cr.sup.+6 /kg)                                                                         in mixing water                                 ______________________________________                                        0.05                          6.4                                             0.10  10.0       9.3          0.3                                             0.25  7.9        7.4          0.0                                             0.50  4.6        3.9                                                          0.75  1.2        2.0                                                          1.00  0.3        0.5                                                          1.50  0.1        0.1                                                          2.00  0.0        0.0                                                          ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                        Water soluble chromate (mg Cr.sup.+6 /kg) after 14 days                       ______________________________________                                        Dosage           Mn(C.sub.2 H.sub.5 O.sub.2).sub.2.4H.sub.2 O                 (%)   MnCl.sub.2.4H.sub.2 O                                                                    (mg Cr.sup.+6 /kg)                                                                           Mn(NO.sub.3).sub.2.4H.sub.2 O                 ______________________________________                                        0.10  2.6        4.2            9.0                                           0.25  0.0        0.0            7.0                                           0.50  0.0        0.0            5.3                                           0.75                            2.4                                           ______________________________________                                                               Manganese (II)                                                                sulfate,                                               Dosage                 technical grade                                        (%)         MnSO.sub.4.H.sub.2 O                                                                     (mg Cr.sup.+6 /kg)                                     ______________________________________                                        0.10        9.4        9.4                                                    0.25        6.7        7.2                                                    0.50        3.5        4.1                                                    0.75        1.9        2.1                                                    1.00        0.5        0.1                                                    1.50        0.1        0.0                                                    2.00        0.0        0.0                                                    ______________________________________                                    

The results show that manganese(II) chloride, manganese(II) acetate,manganese(II) nitrate and dissolved manganese(II) sulfate are able toreduce water soluble chromate in cement.

The results also show that manganese(II) chloride tetrahydrate andmanganese(II) acetate tetrahydrate are more efficient chromate reducingagents than manganese(II) sulfate monohydrate.

The results further show that manganese(II) chloride, manganese(II)acetate and manganese(II) sulfate are stable towards oxidation by air indry cement mixes. Manganese(II) nitrate is stable towards oxidation inpure form but when mixed with cement the results indicate thatmanganese(II) nitrate is partly oxidized by standing with time.

I claim:
 1. A cement composition comprising:1) a cement containingwater-soluble chromate; and 2) at least one manganese(II) compound in anamount sufficient to reduce the amount of water-soluble chromate to atmost 2 mg of Cr(VI) per kg of the cement, wherein the chromate contentis determined by (a) elution of the composition in water for 15 minutesat a water to cement ratio of 1, (b) filtration, and (c) determining thechromate content in the filtrate, and wherein the manganese(II) compoundis selected from the group consisting of (i) a salt with a mineral acidselected from the group consisting of sulfates, hydrogen sulfates,carbonates, nitrates, nitrites, sulfites, sulfides, disulfides,dithionates, thiosulfates, thiocyanates, hydroxides, oxides,monohydrogen phosphates, dihydrogen phosphates and silicates, orhydrates thereof; (ii) a double salt with a mineral acid; (iii) a saltwith an organic acid selected from the group consisting of alkanoicacids, hydroxycarboxylic acids and sulfonic acids: and (iv)manganese(II) oxide sulfate.
 2. A composition as claimed in claim 1,wherein the manganese(II) compound is present in an amount sufficient toreduce the amount of water-soluble chromate to at most 0.5 mg of Cr(VI)per kg of the cement.
 3. A composition as claimed in claim 2, whereinthe manganese(II) compound Is present in an amount sufficient to reducethe amount of water-soluble chromate to at most 0.1 mg of Cr(VI) per kgof the cement.
 4. A composition as claimed in claim 3, wherein themanganese(II) compound is present in an amount sufficient to reduce theamount of water-soluble chromate to at most 0.01 mg of Cr(VI) per kg ofthe cement.
 5. A composition as claimed in claim 1, wherein themanganese(II) compound is present in an amount within the range of from1 to 1000 mmol manganese(II) per kg of cement.
 6. A composition asclaimed in claim 5, wherein the manganese(II) compound is present in anamount within the range of from 5 to 100 mmol manganese(II) per kg ofcement.
 7. A composition as claimed in claim 6, wherein themanganese(II) compound is present in an amount within the range of from10 to 60 mmol manganese(II) per kg of cement.
 8. A composition asclaimed in claim 1, wherein the manganese ( II ) compound is a salt witha mineral acid or with an organic acid.
 9. A composition as claimed inclaim 8, wherein the mineral acid salt is selected from the groupconsisting of sulfate, carbonate, and nitrate.
 10. A composition asclaimed in claim 9, wherein the mineral acid salt is the sulfate.
 11. Acomposition as claimed in claim 10, wherein the manganese(II) sulfate ispresent in an amount within the range of 0.01 to 10% by weight based onthe weight of the cement.
 12. A composition as claimed in claim 11,wherein the manganese(II) sulfate is present in an amount within therange of 0.1 to 1% by weight based on the weight of the cement.
 13. Acomposition as claimed in claim 8, wherein the organic acid salt is asalt with an acid selected from the group consisting of sulfonic acid,alkanoic acid and acetic acid.
 14. A composition as claimed in claim 1,wherein the cement is selected from the group consisting of Portlandcement, belite cement, and slag cement.
 15. A composition as claimed inclaim 14, wherein the cement is a Portland cement.
 16. A composition asclaimed in claim 1, wherein the composition further comprises at leastone material selected from the group consisting of aggregate material,reinforcing material and slaked lime.
 17. A composition as claimed inclaim 16, wherein the aggregate material is selected from the groupconsisting of fine aggregate and coarse aggregate.
 18. A composition asclaimed in claim 1, wherein the composition is in the dry form and themanganese(II) compound is in the form of particles having a meanparticle size below 0.1 mm.
 19. A composition as claimed in claim 18,wherein the composition is in the dry form and the manganese(II)compound is in the form of particles having a mean particle size below0.05 mm.
 20. A composition as claimed in claim 19, wherein thecomposition is in the dry form and the manganese(II) compound is in theform of particles having a mean particle size below 0.02 mm.
 21. Amethod for reducing the amount of water-soluble chromate in a cementcomposition, said method comprising adding to a cement compositioncomprising a water-soluble chromate, a manganese(II) compound in anamount sufficient to reduce the amount of water-soluble chromate to atmost 2 mg of Cr(VI) per kg of cement, and wherein the manganese(II)compound is selected from the group consisting of (i) a salt with amineral acid selected from the group consisting of sulfates, hydrogensulfates, carbonates, nitrates, nitrites, sulfites, sulfides,disulfides, dithionates, thiosulfates, thiocyanates, hydroxides, oxides,monohydrogen phosphates, dihydrogen phosphates and silicates, orhydrates thereof; (ii) a double salt with a mineral acid; (ii) a saltwith an organic acid selected from the group consisting of alkanoicacids, hydroxycarboxylic acids and sulfonic acids; and (iv)manganese(II) oxide sulfate.
 22. A method as claimed in claim 21,wherein the manganese(II) compound is added according to at least one ofthe methods selected from the group consisting of:(i) addition to cementclinker prior to or during grinding; (ii) addition to semi-ground cementclinker; (iii) addition to a component added during the production ofcement; (iv) addition as a concentrated aqueous solution to clinker insuch a manner that the solution is evaporated by adding the solution tothe clinker while the clinker are still warm, or where the clinker areheated mechanically; (v) addition to cement powder prior to or duringtransfer thereof to storage or transport; (vi) addition to cement powderprior to or during packaging thereof; (vii) addition to cement powderduring transfer thereof from a transport vehicle to storage; (viii)addition to cement powder or a composition containing cement powderprior to mixing thereof with water; and (ix) addition as a solution incement mixing water.
 23. A method as claimed in claim 21, whereinmanganese(II) sulfate is added to a cement composition in an amountwithin the range of 0.01 to 10% by weight based on the weight of thecement.
 24. A method as claimed in claim 23, wherein manganese(II)sulfate is added to a cement composition in an amount within the rangeof 0.1 to 1% by weight based on the weight of the cement.
 25. Acomposition as claimed in claim 1, wherein the cement is a pozzolancement.
 26. A composition as claimed in claim 1, wherein the compositionfurther comprises water.
 27. A composition as claimed in claim 1,wherein the composition further comprises cement or concrete additivesselected from the group consisting of a retarding agent, a concreteplasticizer, a concrete superplasticizer and an air entraining agent.28. A cement composition comprising:1) a cement containing water-solublechromate; and 2) at least one manganese(II) compound in an amountsufficient to reduce the amount of water-soluble chromate to at most Imgof Cr(VI) per kg of the cement, wherein the chromate content isdetermined by (a) elution of the composition in water for 15 minutes ata water to cement ratio of 1, (b) filtration, and (c) determining thechromate content in the filtrate, and wherein the manganese(II) compoundselected from the group consisting of (i) a salt with a mineral acidSelected from the group consisting of sulfates, hydrogen sulfates,carbonates, nitrates, nitrites, sulfites, sulfides, disulfidesdithionates, thiosulfates, thiocyanates, hydroxides, oxides,monohydrogen, phosphates, dihydrogen phosphates and silicates, orhydrates thereof; (ii) a double salt with a mineral acid; (iii) a saltwith an organic acid selected from the group consisting of alkanoicacids, hydroxycarboxylic acids and sulfonic acids; and (iv)manganese(II) oxide sulfate.
 29. A composition as claimed in claim 28,wherein the manganese(II) compound is present in an amount within therange of from 1 to 1000 mmol manganese(II) per kg of cement.
 30. Acomposition as claimed in claim 28, wherein the manganese(II) compoundis manganese sulfate.
 31. A composition as claimed in claim 30, whereinthe manganese(II) sulfate is present in an amount within the range of0.01 to 10% by weight based on the weight of the cement.
 32. Acomposition as claimed in claim 28, wherein the cement is selected fromthe group consisting of Portland cement, belite cement, and slag cement.33. A composition as claimed in claim 28, wherein the compositionfurther comprises cement or concrete additives selected from the groupconsisting of a retarding agent, a concrete plasticizer, a concretesuperplasticizer and an air entraining agent.
 34. A composition asclaimed in claim 28, wherein the composition further comprises areinforcing material or slaked lime.
 35. A composition as claimed inclaim 28, wherein the composition is in the dry form and themanganese(II) compound is in the form of particles having a meanparticle size below 0.1 mm.
 36. A composition as claimed in claim 28,wherein the composition further comprises an aggregate material.